1. Field of the Invention
This invention relates to motion dependent video signal processing. More particularly, the invention relates to video standards converters using such signal processing, and to methods of deriving motion vectors representing motion between fields or frames of a video signal.
2. Description of the Prior Art
Video standards converters are well known devices used to convert video signals from one standard to another, for example, from a 625 lines per frame, 50 fields per second standard to a 525 lines per frame, 60 fields per second standard. Video standards conversion cannot be achieved satisfactorily merely by using simple linear interpolation techniques, because of the temporal and vertical alias which is present in a video signal. Thus, simple linear interpolation produces unwanted artefacts in the resulting picture, in particular, the pictures are blurred vertically and judder temporally.
To reduce these problems it has been proposed that video standards converters should use adaptive techniques to switch the parameters of a linear interpolator in dependence on the degree of movement in the picture represented by the incoming video signal.
It has also been proposed, for example for the purpose of data reduction in video signal processing, to generate motion vectors from an incoming video signal by a block matching technique, in which the content of a search block in one field or frame is compared with the respective contents of a plurality of search blocks comprised in a search area in the following field or frame, to determine the minimum difference between the contents so compared, and hence the direction and distance of motion (if any) of the content of the original search block.
Difficulties arise with motion vectors in the case of uncovered/covered surfaces. For example, if a car is seen driving into a tunnel then the car becomes progressively covered while the road and objects behind the car are progressively uncovered. In other words, there are pixels which disappear or appear between one field or frame and the next. A similar situation occurs when there is a scene change.
The present invention is particularly concerned with these problems.